1. Field of the Invention
The present invention relates to a magnetic sensor with a needle-shaped detecting part for use in accurate measurement of presence probability (density) of magnetic substance in an infinite space, and a manufacturing method of a magnetic sensor.
2. Description of the Related Art
For measuring a density of magnetic particles in a fluid or for accurately measuring a concentration of magnetic fluid, a magnetic sensor, having a magnetoresistive effect (MR) element as an element for detecting magnetic field, has been used.
U.S. Pat. No. 7,106,051 B2 discloses an MR sensing device having a substrate with a layer structure for supporting a fluid, to measure a volume density of magnetic particles in the fluid. The layer structure has a first surface area in a first level, a second surface area in a second level different from the first level, and an MR element for detecting the magnetic field of at least one magnetic particle in the fluid. The MR element is positioned near a transition between the first and second surface areas, and faces at least one of the surface areas. U.S. Pat. No. 7,106,051 B2 also discloses that, in the MR sensing device, the fluid containing magnetic particles is supplied over the layer structure, output signal of the MR sensing element is detected while applying a magnetic field, and the output signal from the MR sensing element is compared with a reference signal obtained in the absence of the applied field to determine the volume density of magnetic particles.
Japanese Patent Publication No. 2002-055078A discloses a hypodermic needle-shaped active oxygen sensor with an oxygen reference substance. The oxygen reference substance is separated apart by a solid electrolyte and provided within a sharp tip pipe of the sensor.
Japanese Patent Publication No. 2001-336990A discloses a temperature-measurement apparatus including a needle-shaped sensor with a thermocouple temperature sensing element at its top end.
According to the magnetic sensor described in U.S. Pat. No. 7,106,051 B2, the magnetic field from magnetic particles can be measured with high sensitivity because an MR element is used. However, because the measurement of magnetic field is performed by fluid with a plate-shaped substrate with steps and therefore the shape of the substrate affects the measurement result, it is difficult to accurately measure the density of magnetic particles. That is, if there is a large body such as for example a plate-shaped substrate around a sensing element, because the substrate partially interrupts the magnetic field, an assumption that objects to be detected are uniform in all space does not stand and therefore the measured density is affected. For avoiding this influence, in U.S. Pat. No. 7,106,051 B2, the volume density of magnetic particles is obtained by comparing the output signal from the MR element while applying the magnetic field, to that obtained in the absence of an applied field.
In order to measure the density of magnetic particles accurately without performing such comparison measurement, it is important not only to make the whole magnetic sensor compact but also to reduce the influence from the material and shape of the other body supporting the magnetic detection element as far as possible.
As disclosed in Japanese Patent Publication Nos. 2002-055078A and 2001-336990A, if a detection element is attached to the probe of a fine needle, the sensor can be made compact, and influence from the shape of the substrate can be reduced a little. However, the sensor disclosed in Japanese Patent Publication No. 2002-055078A is an active oxygen sensor in which oxygen reference substance is provided separated apart by a solid electrolyte at its top end of the needle-shaped probe, and the sensor disclosed in Japanese Patent Publication No. 2001-336990A is a temperature sensor in which a thermocouple is provided at its top end of the needle-shaped probe. Namely, these sensors are not magnetic sensors for detecting the density of magnetic particles. Further, each of these needle-shaped probes has an outside diameter of about 0.7 mm at the minimum, and it was difficult in the prior art to achieve a finer probe even for an active oxygen sensor or a temperature sensor. Therefore, even if a magnetic sensor were implemented using a needle-shaped probe as described in Japanese Patent Publication No. 2002-055078A or 2001-336990A, some shape effect would occur because of the size, and it would be difficult to achieve highly accurate detection of the density.